The invention relates to optimization of xerographic process latitude, and more particularly, to the reduction of toner mass in solid area development to improve developer and fuser latitude.
One benchmark in the suitable development by toner particles of a latent electrostatic image on a photoreceptor is the correct relationship of toner concentration in the developer to the tribo electric charge between the carrier and toner. An incorrect relationship, i.e. too much toner concentration, can result in too much background in the developed image. That is, the white background of an image becoming gray. On the other hand, too little toner concentration can result in deletions or lack of toner coverage of the image.
Another benchmark is heat transfer and toner offset (toner sticking to the fuser roll) in the fusing process. The fusing of the toner to a copy sheet takes place when the proper combination of heat, pressure and contact time are provided.
During operation of some fusing systems, the support member is moved through the nip formed by a roll pair. By controlling the heat transfer to the toner, virtually no offset of the toner particles from the copy sheet to the fuser member is experienced under normal conditions. This is because the heat applied to the surface of the fuser member is insufficient to raise the temperature of the surface of the member above the "hot offset" temperature of the toner at which temperature the toner particles in the image areas of the toner liquefy and cause a splitting in the molten toner resulting in "hot offset". Too little heat applied to the surface, however, to eliminate offset can result in insufficient fusing of toner to paper. The amount of heat transferred, therefore, can be neither too high (resulting in toner offset) or too low. Xerographic system latitude is defined by the degree to which the system can properly develop and fuse and image based upon the above variables.
One difficulty with prior art systems has been that to avoid deletions in the processed image, a relatively high toner concentration was required. However, operating at a relatively high toner concentration, the developed mass or toner mass per unit area was beyond the allowable latitude of the fuser resulting in toner offset and unacceptable fused images.
Digital Image manipulation is well known in the prior art. For example, Sharp U.S. Pat. No. 3,573,789 shows a resolution conversion technique by shifting each pixel with surrounding pixels into a resolution expander that automatically produces a multiple number of pixels corresponding to the center pixel. Image enhancement techniques to improve the quality of the image are also well known. For example, Coviello 4,450,483 statistically analyses a pixel with its surrounding pixels to make a determination whether or not the center pixel should remain as a black or white pixel or be changed to either a black or white pixel to improve the quality of the overall image. Walsh Patent 4,437,122 does image enhancement of a digital image by taking each pixel of the digital image and a neighborhood surrounding the pixel and comparing this pattern to a set of reference patterns. Depending upon the match between the pixel and its neighborhood with a particular pattern, the center pixel is expanded into a plurality of predetermined pixels enhancing the overall quality of the image. However, these systems do not consider the problem of potentially conflicting fuser and development latitudes in a xerographic system.
It would be desirable, therefore, to provide a technique to increase xerographic process latitude by the selective reduction of toner in solid area development. It would also be desirable to increase overlapping developer and fuser latitude in the xerographic process by the manipulation of pixels defining the solid area of an image.
It is an object of the present invention therefore to provide a new an improved technique for the selective reduction of toner in solid area development. It is another object of the present invention to selectively alter pixels defining the solid area of an image to improve the xerographic process latitude. It is still another object of the present invention to selectively modulate pixels defining a solid black area of an image to change black pixels to grey Other advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.